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Full text of "Text Book Of Mechanical Engineering"


Appendix VL


allowed at each observation for the extension to reach its
true value, and the lever must be kept floating between
the stops.

10.   Note the maximum load just before the contraction com-

mences appreciably. If possible note the actual con-
traction at this stage.

11.  When  contraction  commences  in  real  earnest   run  the

jockey weight back to zero, and then re-advance it gently
so as to only just balance the stress.

12.  Note the breaking load, which will be generally much less

than the maximum.    Remove the specimen.

Results :

13.   Measure  the   final  dimensions   as   regards   length  over

extreme marks, and at the two inches round the fracture:
also the contracted area.

We are now to state (a) Elastic limit as load and
stress, (b) Maximum load and stress, (c) Extension per
cent in 10 ins. and in the 2 ins. at fracture, (d) Reduction
of area per cent., (e) Modulus of elasticity. The first
step is to carefully plot the stress-strain diagram in two
parts, one shewing the whole life of the bar, and the
other shewing the elastic stage up to yield point with a
large scale of extension.

(a)  The Elastic limit may be discovered from the second

diagram, being a little below the yield point, and indicated
by a slight curvature from the straight line. Dividing
the elastic load by the original area of the bar will give
the elastic stress per square inch.

(b)  The Maximum load is found from the first diagram as

well as from the figure obtained during experiment, and
the maximum unit stress may be expressed both in terms
"of the original area as well as of the area as contracted
on the measurement of this load.

(c)  The Extension per cent, is found both on 10 ins. and

on the 2 ins. of fracture, as

length after fracture — original length
original length

x 100